Asynchronous Transfer Mode (ATM) is being widely used as the preferred transport protocol in broadband integrated services networks. The fixed length ATM cell is particularly suited to multimedia applications including voice, video and data transmission. However, the transparent transport of legacy local area network (LAN) traffic over ATM poses the problem of end-to-end performance. One such performance measure is the achievable throughput, or the amount of data packets successfully transferred between the communicating end-points for the duration of the connection. The other measure of performance is fairness to the users of the service. During periods of congestion when input traffic exceeds the system capacity switches apply a congestion control scheme. The goal of a congestion control scheme is to protect the network while allowing the users to achieve the desired level of performance for their applications. When a congestion control scheme is applied in a switch, it is important to make sure it operates fairly among all of the contending ATM connections. In other words, connections which exceed their fair share of the resources of the switch (buffers and bandwidth) should be targeted first. It is important that connections be allowed to use their fair share of the switch buffers and bandwidth.
An example of an ATM switch congestion control mechanism is called Early Packet Discard (EPD). As soon as a switch reaches a congestion state, the switch starts discarding complete packets, as opposed to individual cells, until the switch is relieved of congestion. EPD prevents buffer overflow and the carriage of partial packets, saving therefore valuable network resources. Partial packets, when transmitted by the network, are discarded at the receiver and retransmission is generally requested. The application of the EPD congestion control mechanism, or any other congestion control mechanism, to an ATM switch with dedicated buffers at the output queues is straightforward. However, many switching architectures use shared memory for a more efficient buffer usage by the contending connections in order to take advantage of the connection varying statistical behavior. In this type of switching architecture, applying EPD or any other congestion control mechanism poses the problem of fairness to the contending connections while keeping the output ports at a high level of utilization. The main difficulty lies in the fact that output port queues share the same memory space (the aggregate buffer pool). Each output port can use instantaneously more than the fraction of the shared memory it would have been allocated if the memory was equally split among the output ports.